From: Fujii Masao <masao.fujii@oss.nttdata.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: Andres Freund <andres@anarazel.de>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: ERROR: invalid spinlock number: 0
Date: Tue, 16 Feb 2021 23:47:52 +0900
Message-ID: <efca765d-a523-dd5d-549d-04f8d76837ac@oss.nttdata.com> (raw)
In-Reply-To: <YCtrN5BgwK9JuQyd@paquier.xyz>
References: <7ef8708c-5b6b-edd3-2cf2-7783f1c7c175@oss.nttdata.com>
<YCUpTL8bZHa+9K+0@paquier.xyz>
<79698f73-7dd7-d9e3-5322-4032b4428653@oss.nttdata.com>
<YCowabyl1dwan+JP@paquier.xyz>
<CA+hUKG+tZHpADD_BsPp=c3EiYeenTTx5AxsZVGkv1mA0J3O21w@mail.gmail.com>
<YCpQwdyXErcTRhVX@paquier.xyz>
<20210215212805.dfahbpqqpnl6lgwm@alap3.anarazel.de>
<918a47fc-e1fa-0c99-15e3-59b520455fe0@oss.nttdata.com>
<YCtrN5BgwK9JuQyd@paquier.xyz>
On 2021/02/16 15:50, Michael Paquier wrote:
> On Tue, Feb 16, 2021 at 12:43:42PM +0900, Fujii Masao wrote:
>> On 2021/02/16 6:28, Andres Freund wrote:
>>> So what? It's just about free to initialize a spinlock, whether it's
>>> using the fallback implementation or not. Initializing upon walsender
>>> startup adds a lot of complications, because e.g. somebody could already
>>> hold the spinlock because the previous walsender just disconnected, and
>>> they were looking at the stats.
>
> Okay.
>
>> Even if we initialize "writtenUpto" in WalRcvShmemInit(), WalReceiverMain()
>> still needs to initialize (reset to 0) by using pg_atomic_write_u64().
>
> Yes, you have to do that.
>
>> Basically we should not acquire new spinlock while holding another spinlock,
>> to shorten the spinlock duration. Right? If yes, we need to move
>> pg_atomic_read_u64() of "writtenUpto" after the release of spinlock in
>> pg_stat_get_wal_receiver.
>
> It would not matter much as a NULL tuple is returned as long as the
> WAL receiver information is not ready to be displayed. The only
> reason why all the fields are read before checking for
> ready_to_display is that we can be sure that everything is consistent
> with the PID. So reading writtenUpto before or after does not really
> matter logically. I would just move it after the check, as you did
> previously.
OK.
>
> + /*
> + * Read "writtenUpto" without holding a spinlock. So it may not be
> + * consistent with other WAL receiver's shared variables protected by a
> + * spinlock. This is OK because that variable is used only for
> + * informational purpose and should not be used for data integrity checks.
> + */
> What about the following?
> "Read "writtenUpto" without holding a spinlock. Note that it may not
> be consistent with the other shared variables of the WAL receiver
> protected by a spinlock, but this should not be used for data
> integrity checks."
Sounds good. Attached is the updated version of the patch.
>
> I agree that what has been done with MyProc->waitStart in 46d6e5f is
> not safe, and that initialization should happen once at postmaster
> startup, with a write(0) when starting the backend. There are two of
> them in proc.c, one in twophase.c. Do you mind if I add an open item
> for this one?
Yeah, please feel free to do that! BTW, I already posted the patch
addressing that issue, at [1].
[1] https://postgr.es/m/1df88660-6f08-cc6e-b7e2-f85296a2bdab@oss.nttdata.com
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
Attachments:bugfix_pg_stat_wal_receiver_v3.patch (text/plain, 3.0K) downloaddiff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.cindex 723f513d8b..9ec71238c4 100644--- a/src/backend/replication/walreceiver.c+++ b/src/backend/replication/walreceiver.c@@ -249,7 +249,7 @@ WalReceiverMain(void)
SpinLockRelease(&walrcv->mutex);
- pg_atomic_init_u64(&WalRcv->writtenUpto, 0);+ pg_atomic_write_u64(&WalRcv->writtenUpto, 0);
/* Arrange to clean up at walreceiver exit */
on_shmem_exit(WalRcvDie, 0);
@@ -1325,7 +1325,6 @@ pg_stat_get_wal_receiver(PG_FUNCTION_ARGS)
state = WalRcv->walRcvState;
receive_start_lsn = WalRcv->receiveStart;
receive_start_tli = WalRcv->receiveStartTLI;
- written_lsn = pg_atomic_read_u64(&WalRcv->writtenUpto);
flushed_lsn = WalRcv->flushedUpto;
received_tli = WalRcv->receivedTLI;
last_send_time = WalRcv->lastMsgSendTime;
@@ -1345,6 +1344,14 @@ pg_stat_get_wal_receiver(PG_FUNCTION_ARGS)
if (pid == 0 || !ready_to_display)
PG_RETURN_NULL();
+ /*+ * Read "writtenUpto" without holding a spinlock. Note that it may not be+ * consistent with the other shared variables of the WAL receiver+ * protected by a spinlock, but this should not be used for data integrity+ * checks.+ */+ written_lsn = pg_atomic_read_u64(&WalRcv->writtenUpto);+
/* determine result type */
if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
elog(ERROR, "return type must be a row type");
diff --git a/src/backend/replication/walreceiverfuncs.c b/src/backend/replication/walreceiverfuncs.cindex 69b91a7dab..63e60478ea 100644--- a/src/backend/replication/walreceiverfuncs.c+++ b/src/backend/replication/walreceiverfuncs.c@@ -63,6 +63,7 @@ WalRcvShmemInit(void)
MemSet(WalRcv, 0, WalRcvShmemSize());
WalRcv->walRcvState = WALRCV_STOPPED;
SpinLockInit(&WalRcv->mutex);
+ pg_atomic_init_u64(&WalRcv->writtenUpto, 0);
WalRcv->latch = NULL;
}
}
diff --git a/src/test/regress/expected/sysviews.out b/src/test/regress/expected/sysviews.outindex 81bdacf59d..6d048e309c 100644--- a/src/test/regress/expected/sysviews.out+++ b/src/test/regress/expected/sysviews.out@@ -83,6 +83,13 @@ select count(*) = 1 as ok from pg_stat_wal;
t
(1 row)
+-- We expect no walreceiver running in this test+select count(*) = 0 as ok from pg_stat_wal_receiver;+ ok +----+ t+(1 row)+
-- This is to record the prevailing planner enable_foo settings during
-- a regression test run.
select name, setting from pg_settings where name like 'enable%';
diff --git a/src/test/regress/sql/sysviews.sql b/src/test/regress/sql/sysviews.sqlindex b9b875bc6a..dc8c9a3ac2 100644--- a/src/test/regress/sql/sysviews.sql+++ b/src/test/regress/sql/sysviews.sql@@ -40,6 +40,9 @@ select count(*) >= 0 as ok from pg_prepared_xacts;
-- There must be only one record
select count(*) = 1 as ok from pg_stat_wal;
+-- We expect no walreceiver running in this test+select count(*) = 0 as ok from pg_stat_wal_receiver;+
-- This is to record the prevailing planner enable_foo settings during
-- a regression test run.
select name, setting from pg_settings where name like 'enable%';
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To: pgsql-hackers@postgresql.org
Cc: masao.fujii@oss.nttdata.com, michael@paquier.xyz, andres@anarazel.de, thomas.munro@gmail.com
Subject: Re: ERROR: invalid spinlock number: 0
In-Reply-To: <efca765d-a523-dd5d-549d-04f8d76837ac@oss.nttdata.com>
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